Allow overriding build timestamp from env

[kaidokert]

Description

Small tweak to build scripts to allow much faster cached builds with ccache and other cache drivers. Local cache such as ccache does not work if -DMBED_BUILD_TIMESTAMP=xy is passed with different value every instantiation. Allowing override through env to a fixed value enables speeding up the local builds substantially.

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Breaking change

[bridadan]

I really like this solution!

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3376
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8359/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 3011
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8359/

[mbed-ci]

Test : SUCCESS

Build number : 3181
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8359/3181

[cmonr]

/morph mbed2-build